This invention relates to duplex optical communication module units and duplex optical communication module devices using these module units.
The optical communication systems include simplex or unilateral communication systems and duplex or bilateral communication systems. In the simplex or unilateral optical communication system, a transmitting station has a light-emitting element such as LED (light-emitting diode) or LD (laser diode) for converting an electric signal to an optical signal, while a receiving station has PD (photodiode) or an APD avalanche photodiode) for converting the optical signal into electric signal. The two stations are coupled together by an optical fiber, and communication is done unilaterally from the transmitting station to the receiving station. In the duplex or bilateral optical communication system in which each of two communicating stations transmit and receive signal to and from the other, a light-emitting element and a light-receiving element are provided in each station. The light-emitting element and the light-receiving element in one station are respectively coupled to the light-receiving elements and the light-emitting element in the other station via respective optical fibers.
The light-emitting and light-receiving elements are assembled respectively in transmitting and receiving module units. The transmitting and receiving module units are assembled in an optical communication module device.
FIG. 1 shows a prior art transmitting module unit. It has a light-emitting element 2 and an IC chip 4 as a circuit for driving the element 2. The light-emitting element 2 and IC chip 4 are secured to a lead frame 6, connected together by wire leads 8 and molded in a transparent plastic material 10. Although not shown, a prior art receiving module unit has a similar structure, which has a light-receiving element and an IC chip as a receiver circuit for amplifying and reproducing the output signal of the light-receiving element. The light-receiving element and IC chip are secured to a lead frame, connected together by wire leads and molded in a transparent plastic material. The transmitting and receiving module units are assembled in a bilateral module device, for instance, as shown in FIGS. 2 and 3. The device comprises an opaque or light-impermeable plastic molding 16 having recesses 12 and 14 for receiving optical connectors for signal reception and transmission and receiving and transmitting module units 18 and 20 received in predetermined recesses of the molding 16 and secured to the recesses by an adhesive 22. Since the transmitting section or transmitting module unit and the receiving section or receiving module unit are packed in a single plastic molding 16, the bilateral module device can be readily handled in operation. In addition, when a plug having two juxtaposed optical fibers is used, the plug may be readily coupled to the bilateral module device.
However, the bilateral module device has the following drawbacks.
(1) The receiving module unit deals with very small signal of the order of several hundred nA and is susceptible to the influence of external noise. On the other hand, in the transmitting module unit provided adjacent to the receiving module unit, the light-emitting element will carry as large current as several ten mA. Noise, therefore, is generated with the switching of this large current, and is coupled to the adjacent receiving module unit to cause erroneous operation thereof.
(2) The transmitting and receiving module units have low reliabilies, especially moisture resistance characteristics, because the module units are molded in a transparent plastic material. A pressure cooker test under conditions of 121.degree. C. and 2 atom. showed that a trouble occurred in the module in several hours.
(3) The light-emitting and light-receiving elements must be efficiently optically coupled to the optical fiber in order to avoid or reduce attenuation of light. This requires precise alignment of the optical axis of each element with that of the optical fiber axis. However, a slight warping or deformation of the lead frame on which each element is mounted would cause a shift of the position of the optical axis of each element, leading to a deviation from the alignment of the optical axes. The deviation from the alignment of the two optical axes may also be caused from the dimensional difference between the module units and a plug having two juxtaposed optical fibers because the module units are independently fixed in the light-impermeable plastic molding 16.